1. Field of the Invention
This invention relates in general to continuous rolling mills producing hot rolled long products such as bars, rods and the like, and is concerned in particular with a method and apparatus for decelerating and temporarily accumulating such products at a selected stage in the hot rolling process.
2. Description of the Prior Art
In the typical rolling mill installation, billets are heated to an elevated rolling temperature in a furnace. The heated billets are then subjected to continuous rolling in successive roughing, intermediate and finishing sections of the mill, with each mill section being comprised of multiple roll stands. For larger products, the entire mill can usually be operated at or close to the maximum capacity of the furnace. However, when the rolling schedule calls for smaller products, the capacity of the finishing section is often reduced to well below that of the furnace and the roughing and intermediate mill sections. Under these circumstances, the roughing and intermediate sections can be slowed to match the capacity of the finishing section, but there are limits beyond which this becomes impractical. This is because acceptable rolling procedure dictates that the heated billets should be introduced into the first stand of the roughing section at a minimum take in speed of not lower than about 0.09–0.1 m/s. Slower take in speeds will likely cause fire cracking of the work rolls.
In other cases, for example, when rolling high speed tool steels or nickel based alloys, a higher take in speed is required to avoid excess cooling of the billet, while lower finishing speeds are required to avoid excessive heat generation, which can cause core melting and surface cracking of the product.
In an exemplary modern day continuous rolling operation, with a furnace capacity of 100–150 tons/hr or greater, a nominal carbon low alloy steel billet with a 150 mm square cross section and a length of 11.7 m is rolled into a 2000 kg. coil. When rolling 5.5 mm diameter rod at the mill's maximum delivery speed of, say, 105 m/s, the take in speed is 0.111 m/sec, which is safely above the acceptable minimum speed. Under these conditions, the mill can produce 64.42 tons/hr (taking into consideration gap and yield). However, if the rolling schedule calls for a 3.5 mm diameter rod, the take in speed for the same size billet at the same maximum delivery speed would have to be lowered to an unacceptably low level of 0.045 m/s, with a corresponding reduction in the mill's tonnage rate to 26.8 tons/hr.
Alternatively, in order to overcome the unacceptably low take in speed, a smaller billet of the same length with, for example, a 106 mm square cross section could be rolled at the maximum delivery speed of 105 m/s and at a safe take in speed of 0.09 m/s. However, this would require a new pass design for the roll stands, different guides, a lowering of the coil weight of the finished product to 1031 kg, and a reduced production rate of 26.31 tons/hr, again taking into consideration gap and yield. The necessity to store different size billets would create further problems.
There exists a need, therefore, for a method and apparatus that will make it possible to roll smaller size products while maintaining the mill take in speeds at or above acceptable minimums, without having to reduce the size of the billets being processed, and preferably while continuing to roll at the mill's maximum tonnage rate.
One prior attempt at achieving this objective is disclosed in U.S. Pat. No. 3,486,359 (Hein), where a laying head temporarily accumulates hot rolled products exiting from the intermediate mill section on a storage reel. The accumulated product is then unwound from the storage reel at a reduced speed for continued rolling in a mill finishing section. A number of drawbacks are associated with the Hein approach. For example, the product is not decelerated prior to being wound onto the storage reel. This, coupled with a lack of control over how the windings are distributed along the reel surfaces, can cause the windings to overlap one another, and this in turn can disrupt the unwinding process.
Also, with the Hein arrangement, the laying head cannot be operated continuously, but instead must be brought to a complete stop at the beginning of each storage cycle so that the product front end can be directed past the storage reel to a downstream stationary pinch roll unit. Thus, during the time required to overcome system inertia and to bring the laying head back up to its operating speed, an unsteady state exists, which can further disrupt the pattern of windings on the storage reel.
The present invention provides an improved method and apparatus for decelerating and temporarily accumulating hot rolled products that differ from the Hein approach in important respects that eliminate the above described drawbacks.